The invention concerns a method for examining structures on a semiconductor substrate. The structures can be, in particular, operable integrated and nonintegrated electronic circuits in the micrometer and nanometer range, or micromechanical components in the micrometer and nanometer range.
The structures of semiconductor elements, for example memory modules, microprocessors, and logic modules, are manufactured on a semiconductor substrate, called a “wafer,” that generally comprises a silicon single crystal and usually has a thickness of between 200 and 600 μm. There are also special wafers having very thin semiconductor substrates, whose thicknesses are much less than 100 μm.
Semiconductor components, which are located in layers a few micrometers thick that are very close to the surface and whose structures can have lateral extensions of, at present, approximately 200 nm or less, are produced on such wafers using lithography and coating and doping methods. In order to manufacture a usable semiconductor chip therefrom, the structures produced in this fashion must be equipped with electrical leads, which in turn can be generated by lithography. The leads generally are made of metals such as, for example, copper or aluminum. Tungsten and tantalum are also used. To allow such conductor paths to cross one another without electrical short-circuiting, intermediate steps are used to apply electrically insulating intermediate layers whose lateral extension and shape can once again be dimensioned very accurately using lithography. Later, once again using various methods, electrical connections must then be produced in the vertical direction; these create contact, through the insulator, between leads or doped zones of one plane and the leads of the other plane. The result is thus a three-dimensional conformation of semiconducting, conducting, and insulating structures, which must be accurately coordinated with one another in terms of their spatial positions.
If defects occur in such structures, they must be examined. This is done, for example, with microscopes. Light microscopes, scanning electron microscopes, atomic force microscopes (AFMs), and acoustic microscopes can be used.
A disadvantage of optical microscopes in this context is that these microscopes are operating close to their resolution limit, since the size of the structures being examined and the light wavelength (which physically limits the resolution of a microscope) are approximately equal in size.
Attempts are occasionally made, for example, to examine the adhesion of electronic structures to the substrate using acoustic microscopes, exploiting the different reflectivity of the sound waves at such defect sites. Once again, however, the limited resolution of the acoustic microscopes constitutes an impediment.
Other examinations can be made with an electron microscope if the structures being examined are first exposed, for example, by chemically removing the substrate that carries the structures. A method of this kind is thus not nondestructive, and requires careful preparation of the layered structures that are to be exposed. In addition, the radiation election beam of an electron microscope can pass through only very thin layers, and three-dimensional tomographic reconstruction of more deeply extending structures is not possible.
Modern computer chips contain a large number of transistors, which are connected to one another by fine wires (called “conductor paths”) made of aluminum. If a chip of this kind is viewed under a light microscope at sufficient magnification, it is even possible to recognize grain boundaries in these conductor paths.
The conductor paths in modern microelectronic components can carry very high current densities (106 A/cm2 and more) without excessive heating, since they are effectively cooled by being embedded into the surrounding silicon or dielectric layers. The current densities are so high that many electrons strike the atoms directly, and can physically displace them in the direction of current flow; this effect is called “electromigration,” and can result in damage to or destruction of the conductor paths.
Since the atoms in grain boundaries or in interfaces between the metal of the conductor paths and surrounding materials can move particularly easily, it may happen that holes form at certain locations due to the high level of material transport, and that at other locations material is pushed out of the conductor path. Either can result in breakdown of the conductor path and thus failure of the entire chip. In the foreseeable future this effect could limit the further miniaturization of computer chips, and materials scientists throughout the world are therefore working very intensively on this problem.
Electromigration in conductor paths is one of the principal causes of the failure of integrated circuits, and because of the ever-increasing integration of such circuits it continues to be a major problem. To study the service life of conductor path metallizations, highly accelerated tests are performed under exaggerated stress conditions, such as elevated current density and elevated temperature. Unpassivated conductor paths are often used to study the migration processes, since they allow better microscopic examination of the failure sites with particularly high spatial resolution. Omission of the passivation applied over the conductor paths—i.e. the protective insulation layers made of, for example, SiO2, Si3N4, or plastic that are applied over the conductor paths—generally causes a change in the way the electromigration processes occur. These changes have to do with material transport and thus with local volume changes, by way of which the pressure and temperature conditions in the vicinity of the conductor path are influenced by the passivation layers. Unpassivated conductor paths, in which the overlying insulation layers are absent, therefore generally behave (e.g. in terms of electromigration) slightly differently from the passivated components used in practice.
The need, therefore, exists for an imaging method that permits high spatial resolution even when the structures to be examined lie under dielectric materials several μm thick. The resolution achievable with microscopes for visible light is often too low. With transmission electron microscopes, high-resolution images can be obtained only of layers up to a maximum of 1 μm thick. Surface-sensitive methods such as atomic force microscopy (AFM) or secondary electron microscopy (SEM) either require destructive specimen preparation or achieve only poor resolution as a result of electron scattering in thick passivation layers. Atomic force microscopes have an additional disadvantage that they are scanning systems and thus require relatively long examination times, and, therefore cannot produce real-time images for continuous observation.
In principle, it is also possible to examine thin metal structures with an X-ray microscope. X-ray microscopes operate in the wavelength region below a maximum of 20 nm. Since the maximum possible resolution of a microscope is in the order of half the wavelength, it is possible to achieve much higher resolution with an X-ray microscope than with a microscope using visible light or UV radiation. In general, the shorter the wavelength of the X-radiation used and the thinner the specimen, the greater the ability of the radiation to penetrate through a specimens.
Depending on the wavelength, X-radiation is in some cases considerably attenuated even in the air under standard conditions. Because X-ray microscopes usually have an overall length of several meters, in order to prevent unnecessary losses of radiation through absorption, the radiation always propagates through evacuated chambers until reaching an area close to the specimen. There the radiation passes through a thin window—made, for example, of a thin but pressure-resistant film—into the air under standard conditions, in which the specimen is also located. Placed behind the sample is usually another window into another evacuated chamber in which the imaging X-ray objective is located, and in which the X-radiation is directed to the X-ray detector. The specimen can also, however, be surrounded by a sealable chamber that is filled with an inert gas or evacuated.
The only high-resolution X-ray objectives used today in X-ray microscopes for wavelengths less than 20 nm are zone plates, since only they can provide sufficiently high resolution. They must be operated with monochromatic radiation, however, since the focal length is inversely proportional to wavelength.
X-ray sources for X-ray microscopes are, for example, deflector magnets, wigglers, or undulators of electron-beam storage rings. The radiation from undulators is quasi-monochromatic (wavelength as a function of bandwidth δλ, i.e. λ/δλ, is approximately 100), and is thus directly suitable for X-ray microscopes using zone plates with a very small number of zones (typically 100 zones) as their X-ray objectives; if zone plates with a greater number of zones are used, or if the radiation sources are wigglers or deflector magnets, the X-radiation must in any case also be monochromatized.
In the wavelength region below 20 nm, X-ray microscopes can be operated in amplitude contrast and in phase contrast. Phase contrast is particularly suitable below 2 nm, since it produces much higher contrast than amplitude contrast. This has to do with the optical constants of the refractive index for X-radiation, which are favorable for phase contrast in this wavelength region.
There are two important types of X-ray microscopes: imaging and scanning X-ray microscopes. Imaging X-ray microscopes produce a real image that can be recorded with a camera. To allow an imaging X-ray microscope to operate in phase contrast, a phase plate with an appropriate phase shift must be arranged in the back focal plane of the zone plate used for imaging. Further details of phase contrast have been described in the technical literature.
Scanning X-ray microscopes generate an image by a serial process in which radiation passes through a specimen one point at a time, and an image is generated on a monitor. Because of this serial image recording performed one point at a time, long examination times are necessary, so that continuous observation of a specimen is not possible. Image creation times are within a range of 100 to 1000 seconds. A scanning X-ray microscope of this kind for three-dimensional tomographic reconstruction of a metal connection in an integrated circuit is known from the article by Zachary H. Levine, Andrew R. Kakulin, Sean P. Frigo, Ian McNulty, and Markus Kuhn: “Tomographic reconstruction of an integrated circuit connect,” Applied Physics Letters, Vol. 74, No. 1, pp. 150-152, Jan. 4, 1999. The thickness of the substrate was thinned to a few μm for the examination. δδ